What is Content Management System?

Whether it's an external site for communicating with clients, an intranet for employees to use, or a site for direct client communication and feedback, all Web sites have a common problem—how to manage the content on them. This is a challenge that can often cost organizations large amounts of time and effort. Producing a powerful yet flexible system for these sites that meets ever-changing requirements while growing to meet your company's emerging needs isn't easy.
One simple definition for a Content Management System (CMS) is that it's a system for managing content. This is a rather unhelpful definition, so I'll break it down into smaller parts for a fuller explanation.
I'll start with a broad definition of content: Content is a unit of data with some extra information attached to it. That piece of data could be a Web page, information about an upcoming event, a Microsoft Word document, an image, a movie clip, or any piece of data that has meaning to the organization deploying the system.

All these items are called content, and they all share similar attributes, such as the need to be added or edited by certain users and be published in various ways. A system called workflow controls these attributes. Workflow is logic defined by the organization's business rules, and it describes a system for managing the content.

Historically there has been a difference between document management systems and CMSs, but mostly these two systems have converged. The essential difference is the items being managed; it's often considered that content is any unit of information, and a document refers to something that's created and edited by humans using software such as Microsoft Office.
Take, for example, a book: A book contains many units of data and may require management slightly different from that required by content. However, in most cases, this is a small difference, and products such as Plone are able to manage the small units of a larger piece of content and reassemble them.

With the ubiquitousness of the Web, many CMSs are now classified as Web CMSs, either because they have a Web-based interface or because they focus on a Web-based delivery system over the Internet or an intranet. Plone provides a Web management interface and Web-based delivery system.

The following is one definition of a CMS (http://www.contentmanager.eu.com/history.htm):

A CMS is a tool that enables a variety of (centralized) technical and (decentralized) nontechnical staff to create, edit, manage and finally publish a variety of content (such as text, graphics, video, and so on) whilst being constrained by a centralized set of rules, process, and workflows that ensure a coherent, validated Web site appearance.